The present invention relates to a source driver, an electro-optical device, a projection-type display device, an electronic instrument, and the like.
As a liquid crystal panel (electro-optical device) used for electronic instruments such as a portable telephone or a projection-type display device, an active matrix type liquid crystal panel using a switching element such as a thin film transistor (hereinafter abbreviated as “TFT”) have been known.
It has been considered to be difficult to reduce power consumption when using an active matrix type liquid crystal panel for portable electronic instruments such as a portable telephone. In recent years, power consumption can be sufficiently reduced using an active matrix type liquid crystal panel. An active matrix type liquid crystal panel has attracted attention due to its advantages (i.e., suitable for an increase in number of colors and motion picture display).
A drive signal of a display device is generally subjected to gamma correction corresponding to the grayscale characteristics of the display device in order to achieve high-definition image display. Taking a liquid crystal device as an example, a grayscale voltage which is gamma-corrected to achieve an optimum pixel transmissivity is output based on grayscale data for grayscale display. A source line is driven based on the resulting grayscale voltage.
In recent years, an increase in display image quality has been increasingly desired. Therefore, an increase in the number of grayscales has been desired for a source driver which drives a source line of an electro-optical device. In this case, it is necessary to supply a larger number of grayscale voltages to each output buffer which drives each source line of the electro-optical device.
When integrating a source driver on a semiconductor substrate, a configuration is generally employed in which a plurality of output buffers are arranged along the long side of a semiconductor substrate. Therefore, grayscale voltage signal lines are disposed to extend along the long side of the semiconductor substrate. Therefore, when increasing the number of grayscale voltage signal lines, the layout area (circuit scale) along the short side of the semiconductor substrate which intersects the long side of semiconductor substrate inevitably increases. For example, when the number of bits of grayscale data of each dot is six, the number of grayscale voltage signal lines is 64 (=26). When the number of bits of grayscale data is increased to eight, the number of grayscale voltage signal lines is increased to 256 (=28). As a result, the layout area of the grayscale voltage signal lines increases by four (=28-6).
JP-A-7-306660 discloses technology in which stepwise voltages are generated in order to reduce the number of grayscale voltage signal lines, and a pulse width modulation signal is generated by sampling a desired voltage from the stepwise voltages to achieve halftone representation. However, this technology has advantages in that grayscale representation is limited to the pulse-width modulation method, and it is difficult to increase the image quality when a larger number of grayscales is required.
It is also difficult to set the levels of the stepwise voltages with high accuracy. Even if the levels of the stepwise voltages can be set with high accuracy, the circuit becomes complicated. In particular, it becomes difficult to generate the stepwise voltages of which the levels are set with high accuracy, as disclosed in JP-A-7-306660, as the number of grayscales increases so that the difference in voltage between the grayscales decreases.
A high-definition image display is also desired for a projection-type display device. Although a reduction in power consumption is not desired for a source driver employed for a projection-type display device, a reduction in size of the source driver is particularly desired in order to reduce the circuit scale.